This invention relates to a method of making electronic circuits, and in particular, to a method of making microwave circuits wherein a multiplicity of circuits are formed and tested in an integral structure, which includes a glass substrate, prior to division into individual circuits.
In the manufacture of integrated circuits, the production goals of low costs and high reproducibility are achieved through the use of fabrication processes using photolithographic techniques, whereby a multiplicity of circuits are fabricated in a single manufacturing sequence. This concept, referred to as batch processing, is used in the fabrication of semiconductor devices. The best example of the impact of batch processing in achieving low production costs is found in the silicon integrated circuit industry, where highly complex products are produced with a remarkably low unit cost.
The use of silicon (Si) integrated circuit technology is limited to low frequency applications (generally less than 1 GHz) by the limited electron mobility and low intrinsic resistivity of silicon. The low electron mobility limits the high frequency performance of the active semiconductor elements while the low bulk resistivity characteristic of the material creates high transmission losses at the higher microwave frequencies.
Gallium arsenide (GaAs) is presently used as an alternative semiconductor material for microwave integrated circuits. This material provides excellent results in many applications but it also possesses limitations which preclude its use in other important applications. The most important limitation of GaAs is the high cost of the basic wafer. GaAs wafers typically cost 20 to 100 times the cost of an equivalent silicon product. Further, many microwave circuits require large surface areas to accommodate the passive element structures and rely on extremely fine conductive pattern resolution during fabrication. As a result, the equivalent number of circuits available from a single wafer is often very low when compared with the number of low frequency silicon counter-parts obtained from the same size wafer. The material requirements lead to an unacceptable cost per circuit. In addition, many circuit designs require trimming, alignment or other adjustments after fabrication in order to compensate for variations in the characteristics of the active microwave devices sited in the individual circuits. Thus far there is no demonstrated effective technique, similar to the silicon manufacturing techniques, for accomplishing batch processing with GaAs mircowave integrated circuits.
Electronic components and subsystems are usually comprised of both low frequency and high frequency functions. Low frequency functions are ususally implemented using silicon devices and integrated circuits. High frequency functions in the microwave regime are usually implemented using GaAs diodes, FETs and integrated circuits. Because most applications require several Si and GaAs discrete devices or monolithic circuits, conventional hybrid technology has been used to incorporate these functions onto a single or multiple substrates (alumina, duriod, etc.).
To overcome the limitations associated with conventional hybrid technology, the trend in research has been to increase the level of integration and to investigate the integration of different devices on a single semiconductor substrate. Although impressive progress has been made in every area of investigation, near term applications require new manufacturing techniques for the implementation of ancillary functions associated with the utilization of discrete devices and monolithic circuits fabricated using different semiconductor substrate technologies. Glass substrate technology is capable of serving the above function with low cost and reproducibility features akin to those of monolithic circuit technology. Furthermore, the approach is capable of incorporating optical devices with low frequency and high frequency functions for use in novel opto-electronic components.
The conventional manufacturing technique for the fabrication of high performance hybrid microwave integrated circuits (HMIC) utilizes a low loss dielectric substrate for the placement of passive circuit elements in combination with packaged or unpackaged semiconductor chips thereon. The most commonly used substrate materials are alumina (Al.sub.2 O.sub.3), beryllia (BeO) and fused silica (SiO.sub.2). Most HMIC's utilize discrete chips for the capacitors as well as transistors. Hybrid circuits exhibit excellent performance characteristics and a high degree of flexibility but also require substantial labor for assembly. The high labor content leads to high fabrication cost and introduces unpredictable variations in component placement and bonding. These variations in assembly degrade the circuit's performance and highly skilled technicians are required to tune the rf circuit in order to attain consistent performance standards.
In the development of an improved low cost HMIC manufacturing process, the use of less expensive dielectric substrate materials which do not compromise circuit performance need to be considered. The fabrication process must allow passive circuit elements and their interconnections to be formed by batch processing techniques with high accuracy. Subsequent assembly and test procedures should be compatible with automation. The inability of present HMIC manufacturing techniques to utilize low cost substrates in an accurate and automated batch processing manufacturing sequence has resulted in the high cost of hybrid microwave integrated circuits. Especially when the cost is compared to that of silicon circuits fabricated by batch processing and testing.
Important requirements to be addressed in the fabrication of microwave integrated circuits are as follows: the substrate must have low loss microwave transmission characteristics; the conductive ground plane for the circuit must be accessible through short distances to provide low parasitic inductance to ground; the substrate must exhibit a smooth surface finish in order to provide a base for the fabrication of large area passive components using thin film techniques; and the active semiconductor devices must be mounted on a good thermal conductor to effectively provide adequate heat dissipation.
Present techniques for fabricating HMIC circuits utilize typically a 15-mil thick alumina substrate with a relative dielectric constant of 10. Ground connections are usually provided by small diameter via holes which are electroplated to the ground plane formed on the bottom surface of the substrate. The via holes are formed in the substrate by the use of a laser drilling. Alternatively, the holes can be drilled or punched mechanically in the substrate in the "green" state prior to sintering.
Laser drilling techniques generate splatter onto adjacent surface areas and result in the build-up of slag about the periphery of the hole. The accuracy attainable with laser drilling of small diameter holes is not acceptable for high density circuit fabrication. In addition, optically transparent substrate materials, such as fused silica, are extremely difficult to drill mechanically. In the case of small diameter hole formation while the substrate is in its "green" state, significant tolerance problems have been encountered as the dimensions and internal surface change as the material is sintered. Consequently, present hole formation techniques in substrates have resulted in reducing the yield and increasing the cost of microwave integrated circuits.
A micro-smooth surface is required for the fabrication of thin film capacitors. Since alumina and beryllia are polycrystaline materials with many grain boundaries and other surface defects, to produce a smooth surface, it is necessary to apply an amorphous glaze to the surface of the substrate. However, this approach has limited use due to the complexity of the process and difficulty in controlling the electrical characteristics of the resulting multi-layer dielectric substrate.
The need for a thermally conductive mounting surface for the unpackaged active devices in an HMIC usually requires attachment of active semiconductor devices on a metallic carrier placed below the dielectric substrates. Since the dielectric substrate is usually much thicker than the unpackaged active devices, either a machined metallic carrier or a separate mechanical processing step is necessary to raise the semiconductor device to the circuitry located on the upper surface of the substrate. It is desirable to limit the thickness of the substrate in order to reduce parasitic ground inductances in the circuit. Typically, common substrates with thicknesses on the order of 0.010 inches are very difficult to process due to breakage during handling. As a result, microwave circuits are manufactured today with specifications and techniques which trade off the various physical limitations of the materials against the performance and manufacturing costs.
Accordingly, the present invention addresses a method of manufacture wherein batch processing techniques can be used for the definition and formation of passive structural elements and subsequent assembly and testing of the microwave integrated circuit can be performed in an automated fashion prior to the separation of the substrate into a multiplicity of individual circuits. Thus, the heretofore practiced intermediate step of separating the substrate into individual circuits prior to location and attachment of the die containing the active semiconductor elements is eliminated.
Furthermore, the present method is well-suited for the use of thin substrate materials. In the prior art it is recognized that the thickness of the substrate is dictated in part by the dielectric constant of the substrate material. As the substrate thickness increases, the likelihood that present processing steps utilized for hole formation will result in damage to the substrate surface area proximate to the hole also increases since the time required to effect hole formation increases correspondingly. Also, the time in which undesired lateral effects can take place on the surface of the substrate increases. Accordingly, the present invention is directed to a novel method of making microwave integrated circuits, wherein a relatively thin substrate possessing a low dielectric constant is utilized in order to reduce the processing time required for hole formation. This invention further includes the step of chemically etching holes in the substrate to substantially reduce the undesired effects of present manufacturing techniques.